A prime objective of this invention is to provide a polyurethane coating which is scratch-resistant, even when it is manufactured under temperature conditions of 90.degree.C or higher.
In the copending and commonly assigned application for U.S. patent Ser. No. 369,087 filed June 11, 1973, there is described the use of scratch-resistant plastic coatings on the interior surface of safety glass for automobiles and the like, and in particular the use of weakly cross-linked aliphatic polyurethanes to form such coatings.
According to this prior copending application such coatings should have the following properties:
______________________________________ Modulus of elasticity about 1,000 kg/cm.sup.2 Mean modulus of deformation 200 kg/cm.sup.2 Elongation at break 100% Breaking strength 100 kg/cm.sup.2 Plastic deformation, expressed as percentage of total elongation at break 2% ______________________________________
Such polyurethane coatings are made by curing liquid mixtures comprising two constituents, namely (1) an aliphatic polyether having free (i.e. reactive) hydroxyl groups and (2) an aliphatic polyisocyanate having reactive isocyanate groups. The liquid mixture is cast into a sheet which is then cured, whereby hydroxyl groups of the polyether molecules react with isocyanate groups of the aliphatic isocyanate molecules to form a three-dimensional cross-linked polyurethane structure.
In German published application No. 2,058,504 there is described an exemplary mode of preparing polyurethanes of this type in which component (1) is a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and in which component (2) is a biuret of 1,6-hexamethylene-diisocyanate. In example III of this German application a coating is prepared by curing a mixture of 128 parts by weight of such a biuret together with 100 parts by weight of such a polyglycol ether. Since this application is silent as to the relative percentages of reactive hydroxyls and isocyanates respectively in the two components it is not clearly defined how close to the stoichiometric these proportions are.
The general teaching of the prior art is vague in regard to the stoichiometry desirable in forming polyurethanes from polyols and polyisocyanates. For example, U.S. Pat. No. 3,791,914 states that ordinarily the amount of polyisocyanate utilized is sufficient to react one equivalent of isocyanate per hydroxyl equivalent of the polyol; but that this ratio is not critical "except for obtainment of optimum results, higher or lower amounts being employed without adverse results".
The present applicants have found that stoichiometrically equivalent proportions are disadvantageous when a high temperature such as about 90.degree.C or higher is used in affixing the polyurethane plastic sheet to the glass. Such higher temperature is required particularly when the outer layer of polyurethane is affixed simultaneously with lamination of two or more glass sheets interlayered with polyvinylbutyral. The temperature required for such an assembly is between about 120.degree. and 140.degree.C. During such high heat conditions, the physical properties of the outside polyurethane sheet deteriorate. In particular, the scratch-resistance is substantially reduced.
Specifically, present inventors have observed the above described results when polyurethane coatings were prepared from mixtures of 100 parts by weight of a component (1) being a polyglycol ether resulting from the condensation of propylene oxide with trimethylolpropane and having 10.5-12% free hydroxyls, together with 128 parts by weight of (2) a biuret of 1,6-hexamethylene-diisocyanate having 21-22% isocyanate groups. The weight ratio W.sub.2 /W.sub.1 of components (2) to component (1), equal to 1.28, corresponds approximately to a stoichiometric equivalence between the available isocyanate and hydroxyl groups respectively.
Using a mar test to be described in greater detail below, a coating prepared from this composition by curing at a temperature below 90.degree.C, has a normal scratch-resistance equal to about 30 grams. But after a treatment at 90.degree.C for a half-hour, the scratch-resistance decreases to about 2 grams. This reduction is so great as to make the product unsuitable for use on the interior surface of a safety windshield in a motor vehicle. The surface layer has become too rigid and too readily scratched.